1. Field of the Invention
The present invention relates to an optical pickup apparatus, and more particularly, to an optical pickup apparatus capable of quantitatively detecting a change in thickness of a recording medium.
2. Description of the Related Art
In general, an information recording/reproducing density increases as a size of a light spot formed on a recording medium by an optical pickup apparatus decreases. The size of the light spot decreases, as shown in Mathematical Formula 1, as a wavelength λ of light in use becomes shorter, and an NA (numerical aperture) of an objective lens becomes greater.Size of light spot∝λ/NA.  Mathematical Formula 1
Thus, to decrease the size of the light spot formed on the recording medium in order to achieve a high density recording medium, a short wavelength light source, such as a blue semiconductor laser, and an objective lens having a high NA are necessarily adopted in the optical pickup apparatus. In a field to which the present technology pertains, a format enabling an increase of a recording capacity to 22.5 GB or more by using the objective lens having a 0.85 NA and a reduction of a thickness of the recording medium to 0.1 mm is noted to prevent deterioration of a performance due to inclination of a surface of the recording medium. Here, the thickness of the recording medium denotes a distance from a light input surface to a recording surface of the recording medium.
However, as shown in Mathematical Formula 2, a spherical aberration W40d is proportional to both the NA of the objective lens to the fourth power and an error in thickness of the recording medium. Accordingly, to adopt the objective lens having a high NA of about 0.85, the recording medium must have a uniform thickness within a range of ±3 μm. Nevertheless, it is very difficult to manufacture the recording medium having the thickness of 0.1 mm and within the scope of the above error in thickness.                               W                      40            ⁢            d                          =                                                            n                2                            -              1                                      8              ⁢                              n                3                                              ⁢                                    (              NA              )                        4                    ⁢          Δ          ⁢                                          ⁢                      d            .                                              Mathematical        ⁢                                  ⁢        Formula        ⁢                                  ⁢        2            Here, n is a refractive index of an optical medium of the recording medium.
FIG. 1 is a graph showing a relationship between the error in thickness of the recording medium and an OPD (optical path difference) generated due to the error when the light source emitting a light beam having a wavelength of 400 nm and the objective lens having a 0.85 NA are employed. As shown in FIG. 1, the OPD increases in proportion to the error in thickness. Since the OPD generated by the error in thickness of the recording medium corresponds to the spherical aberration, the error in thickness of the recording medium is reflected to the optical pickup apparatus in a form of the spherical aberration.
Thus, it is necessary to detect the spherical aberration generated due to the error in thickness of the recording medium and correct the detected spherical aberration in a system using the high NA such as 0.85 NA.
FIG. 2 shows an optical arrangement of a conventional optical pickup apparatus disclosed in Japanese Patent Publication No. 2000-155979 which is capable of detecting and correcting the spherical aberration. Referring to FIG. 2, the conventional optical pickup apparatus includes a light source 10, an objective lens 17 focusing a light beam emitted from the light source 10 on a recording medium 1, a half mirror 11 changing a proceeding path of the light beam reflected by the recording medium 1 and passing through the objective lens 17, a hologram 20 splitting the light beam, of which a proceeding path is changed by the half mirror 11 into a first light beam passing through a particular area and a second light beam passing through another area, and deflecting the split light beams, first through fourth photodetectors 21 detecting the first light beam passing through the particular area and deflected by the hologram 20, a signal processing circuit 23 detecting an aberration from detection signals from the first through fourth photodetectors 21, and a wavefront changing device 25 changing a wavefront of the light beam proceeding toward the recording medium 1 from the light source 10 according to a correction signal input from the signal processing circuit 23. Here, reference numeral 13 denotes a collimating lens changing a divergent light beam emitted from the light source 10 to a parallel beam.
FIG. 3 shows the OPD when the spherical aberration is generated. When the spherical aberration is generated, wavefronts 27a and 27b delayed symmetrically to an optical axis c are generated with respect to a reference wavefront 27 at a center of an aperture. In contrast, the spherical aberration is generated in a case in which the wavefront is preceded symmetrical to the optical axis c.
Thus, the hologram 20, as shown in FIG. 4, has first and second diffraction areas 20a and 20b for selecting the light beam in a delayed wavefornt area, dividing the selected light beam into halves with respect to an axis x crossing the optical axis, and diffracting the half light beams in directions symmetrical to each other to proceed toward first and fourth photodetectors 21a and 21b. Also, the hologram 20 includes a third diffraction area 20c diffracting the light beam in an upper area above the axis x of the light beam except for the delayed wavefront area and making the diffracted light beam proceed toward a second photodetector 21b, and a transmission area 20d transmitting the light beam in a lower area below the axis x, as is, to proceed toward a third photodetector 21c. The first and second diffraction areas 20a and 20b are half-ring shaped.
Each of the first and fourth photodetectors 21a and 21d has a two-section structure so that generation of the spherical aberration can be noted when a focus state of the light beam is detected. Also, each of the second and third photodetectors 21b and 21c has a structure divided into two so that a focus error signal can be detected in a knife edge method.
FIGS. 5A through 5C are views showing a change in patterns of the light beams received by the first through fourth photodetectors 21 according to the generation of the OPD. FIG. 5A shows the patterns of light beams received by the first through fourth photodetectors 21 when a delayed wavefront of the light beam is generated. The light beam of the delayed wavefront area, which is diffracted in the first and second diffraction areas 20a and 20b, respectively, is focused behind the first and fourth photodetectors 21a and 21d. Since the patterns of the light beams received by the first and fourth photodetectors 21a and 21d are symmetrical, signals detected in a first section A of the first photodetector 21a and a second section D of the fourth photodetector 21d are greater than that detected in a second section B of the first photodetector 21a and a first section C of the fourth photodetector 21d, respectively.
Referring to FIG. 5B, in which the aberration is not generated, signals having the same amplitude are detected in the first and second sections A and B of the first photodetector 21a and also signals having the same amplitude are detected in the first and second sections C and D of the fourth photodetector 21d. 
FIG. 5C shows the patterns of the light beams received by the first through fourth photodetectors 21 when a preceding wavefront of the light beam is generated. The light beam in a preceding wavefront area which is diffracted in the first and second diffraction areas 20a and 20b is focused in front of each of the first and fourth photodetectors 21a and 21d. The signal detected in the second section B of the first photodetector 21a and the first section C of the fourth photodetector 21d is greater than that detected in the first section A of the first photodetector 21a and the second section D of the fourth photodetector 21d, respectively.
Thus, a spherical aberration signal SES′ is detected by subtracting a first sum signal of a detection signal b of the second section B of the first photodetector 21a and a detection signal c of the first section C of the fourth photodetector 21d from a second sum signal of a detection signal a of the first section A of the first photodetector 21a and a detection signal d of the second section D of the fourth photodetector 21d, as shown in Mathematical Formula 3.SES′=(a+d)−(b+c).  Mathematical Formula 3
By using the conventional aberration detection method, an amount and sign of the aberration with respect to the spherical aberration can be detected.
However, since the conventional aberration detection method does not quantitatively detect a change in thickness of the recording medium, it is difficult to appropriately correct the spherical aberration corresponding to the change in thickness of the recording medium.